avenger of blood (literally the redeemer of blood): The law allowed a victim's family to avenge murder. They could execute the killer when they found him (Numbers 35:16–21).
The idea was that blood shed by the victim must be paid for by the blood of the person responsible for the act (see Genesis 4:15; 9:6). In this way, the land made unclean by the shed blood could be purified (Numbers 35:33–34).